This is a guest post by Rachel Trent, Digital Collections and Automation Coordinator in the Geography and Map Division.
Every time you look at an online image of a historical map, what you’re viewing is really just a spreadsheet of numbers. Or more likely, three spreadsheets, one each for red, green, and blue (the technical way to describe this is as a “3-dimensional array”, but it’s ok to simply think of it as three spreadsheets). Each of the image’s pixels is represented by a number from the red spreadsheet, the green spreadsheet, and the blue spreadsheet. Your device simply visualizes that numerical data as a grid of colors.
Thousands of Library of Congress maps are imaged each month, allowing you to not only view them online but also allowing you to analyze a